The invention relates to a facet mirror device that may be used within an optical device used in exposure processes, in particular in microlithography systems. It further relates to an optical imaging arrangement comprising such a facet mirror device. It further relates to a method of supporting a facet element of a facet mirror device and a method of manufacturing a facet mirror device. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical element modules comprising optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units. Facet mirror devices as the ones mentioned above, among others may serve to homogenize the illumination light beam (illuminating the mask), i.e. to effect a power distribution within the illumination light beam which is as uniform as possible.
Due to the ongoing miniaturization of semiconductor devices there is not only a permanent need for enhanced resolution but also a need for enhanced accuracy of the optical systems used for fabricating those semiconductor devices. This accuracy obviously not only has to be present initially but has to be maintained over the entire operation of the optical system. A particular problem in this context is proper heat removal from the optical components to avoid uneven thermal expansion of these components leading to uneven deformation of these components and, ultimately, to undesired imaging errors.
As a consequence highly sophisticated facet mirror devices have been developed such as they are disclosed, for example, in DE 102 05 425 A1 (Holderer et al.) and DE 103 24 796 A1 (Roβ-Meβemer), the respective entire disclosure of which is incorporated herein by reference.
Both these documents, among others, show facet mirror devices where facet elements with a spherical rear surface sit in an associated recess within a support element. The spherical rear surface rests against a corresponding spherical wall of the support element confining this recess. While such a sphere to sphere interface theoretically may provide a large area of contact with good heat transfer from the facet element to the support element, this large area contact mainly depends on the manufacturing accuracy of both, the facet element and the support element. Furthermore, the spherical recess is rather expensive to manufacture at an accuracy of a few microns or less as it is desirable in many cases in all three directions in space.
To overcome the problem of heat transfer DE 103 24 796 A1 (Roβ-Meβemer) suggests to place a relatively soft coating (e.g. a gold coating) onto one of the spherical surfaces which compensates manufacturing tolerances by deformation. However, despite the low rigidity of this coating, due to the large contact area such deformation requires relatively large forces prone to introduce undesired deformation into the facet element.
Another approach is disclosed in DE 102 05 425 A1 (Holderer et al.) wherein the spherical rear surface of the facet element, more or less in a line contact, rests against a conical wall confining the recess receiving the facet element. This solution, due to the line contact provides a lower heat transfer while still not considerably reducing the manufacturing effort necessary for the conical wall to have the accuracy needed for properly positioning the facet element.
A third approach to support the facet elements is disclosed in DE 102 05 425 A1 (Holderer et al.) wherein the spherical rear surface of the facet element, more or less in a three point contact, rests against three small spheres each located at a free end of a support pin element. Here, the heat transfer is even worse while as well not considerably reducing the manufacturing effort necessary for the three small spheres to have the desired accuracy.
In all three cases outlined above, a manipulating lever is connected to the rear surface of the facet element, corresponding manipulators acting on the manipulating lever to adjust the position and, predominantly, the orientation of the facet element with respect to the support element.
Furthermore, in some cases, the manipulating lever is used for fixing the facet element relative to the support element once it has been adjusted,